Final answer:
The Incremental Model is most suited for developing an interactive travel planning system focused on environmental impact due to its iterative nature allowing for gradual integration of features and refinements.
Step-by-step explanation:
For an interactive travel planning system that aims to help users plan journeys with the lowest environmental impact, the most appropriate generic software process model would be C. Incremental Model. This model allows the development team to gradually build the software in pieces, making it easier to integrate various environmental impact factors and user feedback iteratively. As the project progresses, additional features and refinements can be incrementally added and tested, ensuring that the system adapts to the changing requirements of environmental impact assessments and user needs.